Short remainder, the 1959 model has just atmospheric outlet on the left side of the crankcase, just below the cylinder. The 1960 model has outlet in the same place but with two ring/plates operating as one way valve and the small diameter pipe connected back to the oil tank, additionally there is breather in the crankshaft nut connected to the chaincase.
I do have 1959 bike but with 1960 setup.
I did experience increase in overpressure in the chaincase manifesting with lots of oil coming out from the chaincase breather hose.
So I followed advice from certain Constellation owner and blocked off the breather nut. Wrong advice. Don’t do it.
I am old but did a rookie mistake. Mixed two different solutions from different years. Closing off the breather nut perhaps works well with atmospheric outlet. Not with these two tiny valve plates. As a result I have now oil weeping from under cylinders. Suspect crankcase pressure damaged the paper seals.
And with hindsight I suspect original reason for overpressure was one of the plates in the side breather got skewed and didn’t work properly. With engine working amount of overpressure from the side pipe was negligible compared to same setup but with plates removed.
I was very surprised when I recently stripped my Constellation (1960). I found that there were holes, around 10mm diameter, (definitely from the factory) that directly connected both the crank and the timing case with the oil tank. Because of these holes I couldn't see the point of breathing gas though a one way valve from the base of the barrel into the oil tank when there's a hole leading leading back into the crankcase. I have plugged the hole in the top of the oil tank and directed the breather onto the chain.
I've only stopped the one way breather below the left hand barrel breathing into the oiltank. As I said in my last post there are internal holes connecting the crank area with the oil tank and the so the one way valve will reduce the pressure in the crank area, the timing case and the oil tank if it vents to the atmosphere.